Golf ball

ABSTRACT

A golf ball includes a spherical core, and a cover covering the spherical core. The spherical core is formed from a rubber composition including a base rubber, a co-cross-linking agent, a cross-linking initiator, an unsaturated fatty acid and/or a metal salt thereof, and an aromatic carboxylic acid and/or a metal salt thereof, the co-cross-linking agent includes an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof, the unsaturated fatty acid and metal salt thereof excludes an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and a metal salt thereof, and the rubber composition includes a metal compound when the co-cross-linking agent contains only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priority to Japanese Patent Application No. 2016-165645, filed Aug. 26, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a golf ball excellent in flight distance on a driver shot, more specifically, relates to an improvement of a core of a golf ball.

Description of Background Art

Japanese Patent Laid-Open Publication No. 2008-212681 describes a golf ball that includes a cross-linked molded product of a rubber composition as a structural element. Japanese Patent Laid-Open Publication No. 2013-27487 and Japanese Patent Laid-Open Publication No. 2013-27488 describes a golf ball that includes a spherical core and at least one cover covering the spherical core. The spherical core is formed from a rubber composition. The entire contents of these publications are incorporated herein by reference.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a golf ball includes a spherical core, and a cover covering the spherical core. The spherical core is formed from a rubber composition including a base rubber, a co-cross-linking agent, a cross-linking initiator, an unsaturated fatty acid and/or a metal salt thereof, and an aromatic carboxylic acid and/or a metal salt thereof, the co-cross-linking agent includes an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof, the unsaturated fatty acid and metal salt thereof excludes an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and a metal salt thereof, and the rubber composition includes a metal compound when the co-cross-linking agent contains only an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIGURE is a partially cutaway cross-sectional view illustrating a golf ball according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.

A golf ball according to an embodiment of the present invention includes a spherical core and at least one cover covering the spherical core. The spherical core is formed from a rubber composition that contains a base rubber, an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or a metal salt thereof as a co-cross-linking agent, a cross-linking initiator, an unsaturated fatty acid and/or a metal salt thereof (excluding an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or a metal salt thereof), and an aromatic carboxylic acid and/or a metal salt thereof, and further contains a metal compound when only an α,β-unsaturated carboxylic acid having 3-8 carbon atoms is contained as the co-cross-linking agent.

(a) Base Rubber

As the base rubber, a natural rubber and/or a synthetic rubber can be used. For example, a polybutadiene rubber, a natural rubber, a polyisoprene rubber, a styrene polybutadiene rubber, an ethylene-propylene-diene rubber (EPDM) and the like can be used. These rubbers may each be independently used, or two or more of these rubbers may be used in combination. Among these rubbers, particularly preferred is high cis polybutadiene having cis-1,4-bonds, which are advantageous for resilience, in a proportion of 40 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more. A content of the high cis polybutadiene in the base rubber is preferably 50 mass % or more, and more preferably 70 mass % or more.

A 1,2-vinyl bond content of the high cis polybutadiene is preferably 2.0 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less. When the 1,2-vinyl bond content is excessively high, resilience may decrease.

The high cis polybutadiene is preferably synthesized using a rare earth element-based catalyst. In particular, use of a neodymium-based catalyst employing a neodymium compound, which is a lanthanum series rare earth element compound, is preferable in that a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content is obtained with an excellent polymerization activity.

For the high cis polybutadiene, a molecular weight distribution (Mw/Mn) (Mw: weight average molecular weight; Mn: number average molecular weight) is preferably 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and is preferably 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.4 or less. When the molecular weight distribution (Mw/Mn) of the high cis polybutadiene is excessively low, processability may deteriorate; and when the molecular weight distribution (Mw/Mn) of the high cis polybutadiene is excessively high, resilience may decrease. The molecular weight distribution is measured using gel permeation chromatography (“HLC-8120GPC” commercially available from Tosoh Corporation) using a differential refractometer as a detector under conditions of column: GMHHXL (commercially available from Tosoh Corporation), column temperature: 40° C., and mobile phase: tetrahydrofuran, and is a value calculated as a standard polystyrene-converted value.

For the high cis polybutadiene, a Mooney viscosity (ML₁₊₄ (100° C.)) is preferably 30 or more, more preferably 32 or more, and even more preferably 35 or more, and is preferably 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 80 or less. The Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is a value measured according to JIS K6300 using an L rotor under conditions of preheating time: 1 minute, rotor rotation time: 4 minutes, and temperature: 100° C.

As the base rubber, the rubber composition preferably contains at least two high cis polybutadienes each having a Mooney viscosity (ML₁₊₄ (100° C.)) different from each other, and more preferably contains two high cis polybutadienes each having a Mooney viscosity (ML₁₊₄ (100° C.)) different from each other. When two high cis polybutadienes are contained, it is preferable that a first high cis polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of less than 50 and a second high cis polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of 50 or more.

The Mooney viscosity (ML₁₊₄ (100° C.)) of the first high cis polybutadiene is preferably 30 or more, more preferably 32 or more, and even more preferably 35 or more, and is preferably less than 50, more preferably 49 or less, and even more preferably 48 or less. The Mooney viscosity (ML₁₊₄ (100° C.)) of the second high cis polybutadiene is preferable 50 or more, more preferable 52 or more, and even preferable 54 more, and is preferable 100 or less, more preferable 90 or less, even more preferable 80 or less, and most preferable 70 or less.

In the base rubber, a mass ratio ((first high cis polybutadiene)/(second high cis polybutadiene)) of the first high cis polybutadiene to the second high cis polybutadiene is preferably 0.3 or more, more preferably 0.5 or more, and even more preferably 0.7 or more, and is preferably 3.0 or less, more preferably 2.0 or less, and even more preferably 1.5 or less.

As the base rubber, the rubber composition preferably also contains a polybutadiene rubber and a polyisoprene rubber. A Mooney viscosity (ML₁₊₄ (100° C.)) of the polyisoprene rubber is preferably 55 or more, more preferably 60 or more, and even more preferably 65 or more, and is preferably 120 or less, more preferably 110 or less, and even more preferably 100 or less.

In the base rubber, a mass ratio ((polybutadiene rubber)/(polyisoprene rubber)) of the polybutadiene rubber to the polyisoprene rubber is preferably 1 or more, more preferably 2 or more, and even more preferably 4 or more, and is preferably 20 or less, more preferably 15 or less, and even more preferably 10 or less.

(b) Co-Cross-Linking Agent

The α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or metal salt thereof is blended as the co-cross-linking agent in the rubber composition and has an effect of cross-linking rubber molecules by graft polymerization to a base rubber molecular chain. The number of carbon atoms of the α,β-unsaturated carboxylic acid used as the co-cross-linking agent is preferably 3-8, more preferably 3-6, and even more preferably 3 or 4. These α,β-unsaturated carboxylic acids having 3-8 carbon atoms and/or the metal salts thereof may each be independently used, or two or more of them may be used in combination.

Examples of α,β-unsaturated carboxylic acids having 3-8 carbon atoms include facrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like. When the rubber composition contains only an α,β-unsaturated carboxylic acid having 3-8 carbon atoms as the co-cross-linking agent, the rubber composition further contains a metal compound as a component. This is because neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with the metal compound in the rubber composition provides substantially the same effect as using the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-cross-linking agent.

Examples of a metal that forms a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum; and other metal ions such as tin, and zirconium. These metal components can each be independently used or two or more of these metal components can be used in combination. Among them, as the metal component, the divalent metals such as magnesium, calcium, zinc, barium, and cadmium are preferred. This is because, by using a divalent metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, metal cross-linking between rubber molecules is likely to occur. In particular, as a divalent metal salt, for a reason that resilience of a resultant golf ball is increased, a zinc salt of an α,β-unsaturated carboxylic acid having 3-8 carbon atoms is preferred, and zinc acrylate is more preferred. When an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and a metal salt thereof are used in combination as a co-cross-linking agent, a metal compound may be used as an optional component.

A content of the α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or the metal salt thereof, with respect to 100 parts by mass of the base rubber, is preferably 15 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 25 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and even more preferably 35 parts by mass or less. When the content of the α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or the metal salt thereof is less than 15 parts by mass, in order for a member formed from the rubber composition to have an appropriate hardness, it may be necessary to increase an amount of the cross-linking initiator (to be described later) and the resilience of the golf ball tends to decrease. On the other hand, when the content of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof exceeds 50 parts by mass, there is a risk that a member formed from the rubber composition may become excessively hard and shot feeling of the golf ball may be deteriorate.

As the α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or the metal salt thereof, those having a surface covered by a saturated fatty acid and/or a metal salt thereof may also be used. In this case, when a neutralization degree (to be described later) is calculated, a cation component and an anion component of the saturated fatty acid and/or the metal salt thereof used in a surface treatment are respectively included in a cation component and an anion component of the component (b).

(c) Cross-Linking Initiator

The cross-linking initiator is blended in order to cross-link the base rubber component. As the cross-linking initiator, an organic peroxide is preferred. Specific examples of the organic peroxide include organic peroxides such as dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide and the like. These organic peroxides may each be independently used, or two or more of these organic peroxides may be used in combination. Among them, dicumyl peroxide is preferably used.

A content of the cross-linking initiator, with respect to 100 parts by mass of the base rubber, is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.7 parts by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, even more preferably 2.0 parts by mass or less, and particularly preferably 0.9 parts by mass or less. When the content of the cross-linking initiator is less than 0.2 parts by mass, a member formed from the rubber composition tends to become excessively soft and the resilience of the golf ball tends to decrease. When the content of the cross-linking initiator exceeds 5.0 parts by mass, in order for a member formed from the rubber composition to have an appropriate hardness, it may be necessary to decrease the amount of the co-cross-linking agent described above, and there is a risk that the resilience of the golf ball may be insufficient or durability of the golf ball may deteriorate.

(d) Unsaturated Fatty Acid and/or Metal Salt Thereof

The unsaturated fatty acid and/or the metal salt thereof is an aliphatic carboxylic acid having at least one unsaturated bond in a hydrocarbon chain and/or a metal salt thereof. The unsaturated fatty acid and/or the metal salt thereof is preferably a monocarboxylic acid. The unsaturated fatty acid and/or the metal salt thereof does not contain the α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or the metal salt thereof, which is used as the co-cross-linking agent.

The reason that the resilience of the spherical core is improved by blending the unsaturated fatty acid and/or the metal salt thereof is thought to be as follows. When the spherical core is molded, the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubber composition graft-reacts with the base rubber to form a graft polymer, thereby cross-linking rubber molecules. Further, the component (b) can form an ion cluster. Therefore, for the component (b) that forms the ion cluster, the grafting reaction can more easily proceed. Here, the component (d) has a double bond and thus can addition-react with a double bond of the component (b). Further, by performing a cation exchange with the ion cluster formed by the component (b), the component (d) can enter into the ion cluster. In this way, the component (d) addition-reacts with the component (b) or enters into the ion cluster, and thereby, the resulting spherical core becomes highly resilient.

The number of carbon atoms of the unsaturated fatty acid and/or the metal salt thereof is preferably 4 or more, more preferably 5 or more, even more preferably 8 or more, and particularly preferably 12 or more, and is preferably 33 or less, more preferably 30 or less, even more preferably 27 or less, and particularly preferably 26 or less. When the component (d) is an unsaturated fatty acid having 4 or more carbon atoms and/or a metal salt thereof, the component (d) more easily enters into the ion cluster formed by the component (b), and the resulting spherical core becomes highly resilient. When the component (d) is an unsaturated fatty acid having 33 or less carbon atoms and/or a metal salt thereof, the addition reaction between the component (d) and the component (b) more easily occurs, and the resulting spherical core becomes highly resilient.

The number of carbon-carbon double bonds per unit mass of the unsaturated fatty acid and/or the metal salt thereof is preferably 1.00 mmol/g or more, more preferably 1.50 mmol/g or more, and even more preferably 2.00 mmol/g or more, and is preferably 10.00 mmol/g or less, more preferably 9.00 mmol/g or less, and even more preferably 8.00 mmol/g or less. When the number of carbon-carbon double bonds per unit mass of the component (d) is 1.00 mmol/g or more, the addition reaction between the component (d) and the component (b) more easily occurs, and the resulting spherical core becomes highly resilient. When the number of carbon-carbon double bonds per unit mass of the component (d) is 10.00 mmol/g or less, the component (d) more easily enters into the ion cluster formed by the component (b), and the resulting spherical core becomes highly resilient.

The number of carbon-carbon double bonds of the unsaturated fatty acid and/or the metal salt thereof is preferably 1 or more, and is preferably 4 or less, more preferably 2 or less, and even more preferably 1. When the component (d) is an unsaturated fatty acid having 4 or less carbon-carbon double bonds and/or a metal salt thereof, the addition reaction between the component (d) and the component (b) more easily occurs, and the resulting spherical core becomes highly resilient.

The unsaturated fatty acid and/or the metal salt thereof is preferably an unsaturated fatty acid represented by a chemical formula (1) and/or a metal salt thereof:

(in the chemical formula (1), R¹ represents a hydrogen atom or an alkyl group that has 1-25 carbon atoms and may have a substituent group; R² represents an alkylene group that has 1-25 carbon atoms and may have a substituent group; R³ represents an alkylene group that has 2-25 carbon atoms and may have a substituent group; m represents a natural number of 0-5; when m is 2-5, the multiple R² may be identical to or different from each other).

The alkyl group having 1-25 carbon atoms represented by R¹ may have a branched structure or a cyclic structure. However, a linear alkyl group is preferable. The number of carbon atoms of the alkyl group is preferably 1 or more, more preferably 3 or more, and even more preferably 5 or more, and is preferably 25 or less, more preferably 23 or less, and even more preferably 21 or less. An example of the substituent group of the alkyl group having 1-25 carbon atoms represented by R¹ is a hydroxy group.

The alkylene group having 1-25 carbon atoms represented by R² may have a branched structure or a cyclic structure. However, a linear alkylene group is preferable. The number of carbon atoms of the alkylene group is preferably 1 or more, more preferably 3 or more, and even more preferably 5 or more, and is preferably 25 or less, more preferably 23 or less, and even more preferably 21 or less. An example of the substituent group of the alkylene group having 1-25 carbon atoms represented by R² is a hydroxy group.

The alkylene group having 2-25 carbon atoms represented by R³ may have a branched structure or a cyclic structure. However, a linear alkylene group is preferable. The number of carbon atoms of the alkylene group is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more, and is preferably 25 or less, more preferably 23 or less, and even more preferably 21 or less. An example of the substituent group of the alkylene group having 2-25 carbon atoms represented by R³ is a hydroxy group.

The above m is preferably 3 or less, more preferably 2 or less, even more preferably 1 or less, and particularly preferably 0.

As a compound represented by the chemical formula (1), a compound represented by the following chemical formula (2) or chemical formula (3) is more preferable.

In the chemical formula (2), R¹¹ represents a hydrogen atom or an alkyl group that has 1 to 25 carbon atoms and may have a substituent group; and R¹² represents an alkylene group that has 2 to 25 carbon atoms and may have a substituent group.

The alkyl group having 1-25 carbon atoms represented by R¹¹ may have a branched structure or a cyclic structure. However, a linear alkyl group is preferable. An example of the substituent group of the alkyl group having 1-25 carbon atoms represented by R¹¹ is a hydroxy group. The alkylene group having 2-25 carbon atoms represented by R¹² may have a branched structure or a cyclic structure. However, a linear alkylene group is preferable. An example of the substituent group of the alkylene group having 2-25 carbon atoms represented by R¹² is a hydroxy group.

In the chemical formula (2), when R¹¹ is an alkyl group, a ratio (R¹¹/R¹²) of the number of carbon atoms of R¹¹ to the number of carbon atoms of R¹² is preferably 0.1 or more, more preferably 0.5 or more, even more preferably 0.8 or more, and is preferably 10.0 or less, more preferably 5.0 or less, and even more preferably 1.3 or less. When the ratio (R¹¹/R¹²) of the numbers of carbon atoms is within the above range, the addition reaction between the component (d) and the component (b) efficiently occurs, and the resulting spherical core becomes highly resilient.

In the chemical formula (3), R²¹ represents a hydrogen atom or an alkyl group that has 1 to 25 carbon atoms and may have a substituent group; R²² represents an alkylene group that has 1 to 25 carbon atoms and may have a substituent group; and R²³ represents an alkylene group that has 2 to 25 carbon atoms and may have a substituent group.

The alkyl group having 1-25 carbon atoms represented by R²¹ may have a branched structure or a cyclic structure. However, a linear alkyl group is preferable. An example of the substituent group of the alkyl group having 1-25 carbon atoms represented by R²¹ is a hydroxy group. The alkylene group having 1-25 carbon atoms represented by R²² may have a branched structure or a cyclic structure. However, a linear alkylene group is preferable. An example of the substituent group of the alkylene group having 1-25 carbon atoms represented by R²² is a hydroxy group. The alkylene group having 2-25 carbon atoms represented by R²³ may have a branched structure or a cyclic structure. However, a linear alkylene group is preferable. An example of the substituent group of the alkylene group having 2-25 carbon atoms represented by R²³ is a hydroxy group.

The number of carbon atoms of the alkylene group represented by R²² is preferably 25 or less, more preferably 20 or less, and even more preferably 15 or less.

As the alkylene group represented by R²², a methylene group and an ethylene group are preferable, and a methylene group is more preferable.

In the chemical formula (3), when R²¹ is an alkyl group, a ratio (R²¹/R²³) of the number of carbon atoms of R²¹ to the number of carbon atoms of R²³ is preferably 0.1 or more, more preferably 0.5 or more, even more preferably 0.8 or more, and is preferably 10.0 or less, more preferably 5.0 or less, and even more preferably 1.3 or less. When the ratio (R²¹/R²³) of the numbers of carbon atoms is within the above range, the addition reaction between the component (d) and the component (b) efficiently occurs, and the resulting spherical core becomes highly resilient.

The unsaturated fatty acid and/or the metal salt thereof is preferably a linear unsaturated fatty acid and/or a metal salt thereof: The unsaturated fatty acid and/or the metal salt thereof is preferably an unsaturated fatty acid having a carbon-carbon double bond at a terminal of a hydrocarbon chain and/or a metal salt thereof, and/or an unsaturated fatty acid having at least one cis-isomerized carbon-carbon double bond and/or a metal salt thereof, and is more preferably an unsaturated fatty acid having at least one cis-isomerized carbon-carbon double bond and/or a metal salt thereof. When a carbon-carbon double bond is present at a terminal of a hydrocarbon chain, or when at least one cis-isomerized carbon-carbon double bond is present, reactivity of the addition reaction between the component (d) and the component (b) is high, and the resulting spherical core becomes highly resilient.

When the component (d) is an unsaturated fatty acid having 8 or more carbon atoms and/or a metal salt thereof, the component (d) preferable has a first carbon-carbon double bond at a second or later carbon atom, more preferably at a fourth or later carbon atom, and even more preferably at a sixth or later carbon atom, counted from a carboxyl group, which is an unsaturated fatty acid terminal.

When the unsaturated fatty acid and/or the metal salt thereof is an unsaturated fatty acid having 5 or more carbon atoms and/or a metal salt thereof, the unsaturated fatty acid and/or the metal salt thereof preferably has a first carbon-carbon double bond at a fourth or later carbon atom, more preferably at a sixth or later carbon atom, and even more preferably at an eighth or later carbon atom, counted from a carboxyl group side thereof. When the component (d) is an unsaturated fatty acid having a first carbon-carbon double bond at a fourth or later carbon atom counted from a carboxyl group side there, and/or a metal salt thereof, the addition reaction between the component (d) and the component (b) more easily occurs, and the resulting spherical core becomes highly resilient.

Specific examples of the unsaturated fatty acid that forms the unsaturated fatty acid and/or the metal salt thereof (IUPAC name) include butenoic acid (C4), pentenoic acid (C5), hexenoic acid (C6), heptenoic acid (C7), octene acid (C8), nonenic acid (C9), decenoic acid (C10), undecenoic acid (C11), dodecenoic acid (C12), tridecenoic acid (C13), tetradecenoic acid (C14), pentadecenoic acid (C15), hexadecenoic acid (C16), heptadecenoic acid (C17), octadecenoic acid (C18), nonadecenoic acid (C19), icosenoic acid (C20), henicosenoic acid (C21), docosenoic acid (C22), tricosenoic acid (C23), tetracosenoic acid (C24), pentacosenoic acid (C25), hexacosenoic acid (C26), heptacosenoic acid (C27), octacosenoic acid (C28), nonacosenoic acid (C29), triacontic acid (C30), hentria carboxylic acid (C31), dotriacontic acid (C32), tritriacontentic acid (C33), and the like.

Specific examples of the unsaturated fatty acid that forms the unsaturated fatty acid and/or the metal salt thereof (common name) include: unsaturated fatty acids each having a double bond at a terminal thereof such as 4-pentenoic acid (C5, monounsaturated fatty acid), 5-hexenoic acid (C6, monounsaturated fatty acid), 6-heptenoic acid (C7, monounsaturated fatty acid), 7-octenoic acid (C8, monounsaturated fatty acid), 8-nonenoic acid (C9, monounsaturated fatty acid), 9-decenoic acid (C10, monounsaturated fatty acid), and 10-undecylenic acid (C11, monounsaturated fatty acid); unsaturated fatty acids each having a double bond at a site other than a terminal thereof such as myristoleic acid (C14, cis-9-monounsaturated fatty acid), palmitoleic acid (C16, cis-9-monounsaturated fatty acid), stearidonic acid (C18, 6, 9, 12, 15-tetra unsaturated fatty acid), vaccenic acid (C18, cis-11-monounsaturated fatty acid), oleic acid (C18, cis-9-monounsaturated fatty acid), elaidic acid (C18, trans-9-monounsaturated fatty acid), linoleic acid (C18, cis-9-cis-12-diunsaturated fatty acid), α-linolenic acid (C18, 9, 12, 15-tri unsaturated fatty acid), γ-linolenic acid (C18, 6, 9, 12-tri unsaturated fatty acid), gadoleic acid (C20, cis-9-monounsaturated fatty acid), eicosenoic acid (C20, cis-11-monounsaturated fatty acid), eicosadienoic acid (C20, cis-11-cis-14-diunsaturated fatty acid), arachidonic acid (C20, 5, 8, 11, 14-tetraunsaturated fatty acid), eicosapentaenoic acid (C20, 5, 8, 11, 14, 17-penta unsaturated fatty acid), erucic acid (C22, cis-13-monounsaturated fatty acid), docosahexaenoic acid (C22, 4, 7, 10, 13, 16, 19-hexa unsaturated fatty acid), and nervonic acid (C24, cis-15-monounsaturated fatty acid); an unsaturated fatty acid having hydroxy group such as ricinoleic acid (C18, cis-9-monounsaturated fatty acid); and the like.

Among them, preferable examples of the unsaturated fatty acid that forms the unsaturated fatty acid and/or the metal salt thereof include undecylenic acid (C11, monounsaturated fatty acid), myristoleic acid (C14, monounsaturated fatty acid), palmitoleic acid (C16, monounsaturated fatty acid), oleic acid (C18, monounsaturated fatty acid), linoleic acid (C18, diunsaturated fatty acid), eicosenoic acid (C20, monounsaturated fatty acid), erucic acid (C22, monounsaturated fatty acid), nervonic acid (C24, monounsaturated fatty acid), and ricinoleic acid (C18, cis-9-monounsaturated fatty acid).

Examples of the metal that forms the unsaturated fatty acid and/or the metal salt thereof include monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum; and other ions such as tin, and zirconium. These metal components can each be independently used or two or more of these metal components can be used in combination. Among them, as the metal component, the divalent metals such as magnesium, calcium, zinc, barium, and cadmium are preferred, and zinc is particularly preferred. When a divalent metal salt of the unsaturated fatty acid is used, the cation exchange between the component (d) and the ion cluster formed by the component (b) more easily occurs, and the resulting spherical core becomes highly resilient. These unsaturated fatty acids and/or metal salts thereof may each be independently used, or two or more of them may be used in combination.

A content of the unsaturated fatty acid and/or the metal salt thereof, with respect to 100 parts by mass of the base rubber, is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, and is preferably 35 parts by mass or less, more preferably 30 parts by mass or less, even more preferably 25 parts by mass or less, particularly preferably 20 parts by mass or less, and most preferably 17 parts by mass or less. When the content of the component (d) is 1 part by mass or more, an effect of adding the component (d) is sufficiently achieved, and the resulting spherical core becomes highly resilient. When the content of the component (d) is 35 parts by mass or less, the spherical core does not become excessively soft, and the durability and high resilience of the golf ball are not impaired.

(e) Aromatic Carboxylic Acid and/or Metal Salt Thereof

The rubber composition further contains the aromatic carboxylic acid and/or metal salt thereof. By using in combination the unsaturated fatty acid and/or metal salt thereof and the aromatic carboxylic acid and/or metal salt thereof, the flight distance on a driver shot is further improved. The aromatic carboxylic acid has an aromatic ring in this molecule and a carboxy group bonded directly or via an alkylene group (having 1-3 carbon atoms) to the aromatic ring. The aromatic carboxylic acid preferably has a carboxy group directly bonded to the aromatic ring. Aromatic carboxylic acids and/or metal salts thereof may each be independently used, or two or more of them may be sued in combination.

Examples of the aromatic ring include a benzene ring, a condensed benzene ring, and an aromatic heterocycle. Examples of the condensed benzene ring include naphthalene, anthracene, phenalene, phenanthrene, tetracenyl, pyrene and the like. The aromatic heterocycle contains, as atoms forming a ring structure, a carbon atom and at least one atom (hetero atom) other than a carbon atom, and has aromaticity. The aromatic heterocycle may contain one type, or two or more types of hetero atoms. Examples of the hetero atoms include a nitrogen atom, an oxygen atom, a sulfur atom and the like, and among these, an oxygen atom and a sulfur atom are preferable. Further, the number of hetero atoms in the aromatic heterocycle is not particularly limited, but is preferably 2 or less, and more preferably 1. Examples of the aromatic heterocycle include: monocyclic structures such as a five-membered ring structure and a six-membered ring structure; a condensed ring structure; and the like. Among these aromatic heterocycles, a monocyclic ring is preferable, and a five-membered ring is more preferable.

Examples of the aromatic heterocycle include: five-membered ring structures such as pyrrole, furan, thiophene, imidazole, pyrazole, oxazole, isoxazole, and thiazole; six-membered ring structures such as pyridine, pyrazine, pyridazine, pyrimidine, triazine, and tetrazine; and condensed ring structures such as indole, isoindole, benzimidazole, quinoline, isoquinoline, quinoxaline, cinnoline, quinazoline, benzofuran, isobenzofuran, benzothiophene, and benzothiazole.

The number of carboxy groups contained in one molecule of the aromatic carboxylic acid may be 1 (monocarboxylic acid) or 2 or more (polycarboxylic acid), but is preferably 1. In addition to the carboxy group, the aromatic ring may have a substituent group directly bonded to the aromatic ring. Examples of such a substituent group include an alkyl group, an aryl group, an aralkyl group, an alkylaryl group, an amino group that may be substituted, a hydroxy group, an alkoxy group, a halogen group, an acetoxy group, and the like.

Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, and the like. Among these alkyl groups, an alkyl group having 1-6 carbon atoms is preferable, and an alkyl group having 1-4 carbon atoms is more preferable. Examples of the aryl group include a phenyl group, a naphthyl group, an anthryl group, a biphenyl group, a phenanthryl group, a fluorenyl group and the like. Among these aryl groups, a phenyl group is preferable. Examples of the aralkyl group include a benzyl group, a phenethyl group, a phenylpropyl group, a naphthylmethyl group, a naphthylethyl group and the like. Examples of the alkylaryl group include a tolyl group, a xylyl group, a cumenyl group, a mesityl group and the like.

As the amino group that may be substituted, an amino group in which at least one atom of the amino group is substituted with an alkyl group or an aryl group is preferable. Examples of the amino group that may be substituted include a methylamino group, a dimethylamino group, an ethylamino group, a propylamino group, an isopropylamino group, a butylamino group, an isobutylamino group, a tert-butylamino group, a pentylamino group, a hexylamino group, a 2-ethylhexylamino group, a phenylamino group, a diphenylamino group, a naphthylamino group, and the like.

Examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a pentyloxy group, a hexyloxy group, a heptyloxy group, an octyloxy group, a nonyloxy group, a decyloxy group, and the like. Among these alkoxy groups, an alkoxyl group having 1-6 carbon atoms is preferable, and an alkoxyl group having 1-4 carbon atoms is more preferable.

Examples of halogen include fluorine, chlorine, bromine, and iodine.

Specific examples of the aromatic carboxylic acid having a benzene ring to which a carboxy group is directly bonded include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, hememilitic acid (benzene-1,2,3-tricarboxylic acid), trimellitic acid (benzene-1,2,4-tricarboxylic acid), trimeric acid (benzene-1,3,5-tricarboxylic acid), melophanic acid (benzene-1,2,3,4-tetracarboxylic acid), plenitic acid (benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzenehexacarboxylic acid), and the like. Specific examples of the aromatic carboxylic acid having a benzene ring to which a carboxy group is bonded via an alkylene group include α-toluic acid (phenylacetic acid), hydroatropic acid (2-phenylpropanoic acid), hydrocinnamic acid (3-phenylpropanoic acid), and the like.

Examples of the carboxylic acid having a benzene ring substituted with an alkyl group, an aryl group, an amino group, a hydroxy group, an alkoxy group, an acetoxy group or the like include toluic acid (methylbenzoic acid), xylylic acid (dimethylbenzoic acid), plicylic acid (2,3,4-trimethylbenzoic acid), γ-isoduronic acid (2,3,5-trimethylbenzoic acid), durhylic acid (2,4,5-trimethylbenzoic acid), β-isoduronic acid (2,4,6-trimethylbenzoic acid), α-isoduronic acid (3,4,5-trimethylbenzoic acid), vuminic acid (4-isopropylbenzoic acid), 4-tert-butylbenzoic acid, ubitoic acid (5-methylisophthalic acid), biphenyl-4-carboxylic acid, diphenic acid (biphenyl-2,2′-dicarboxylic acid), dimethylaminobenzoic acid, salicylic acid (2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotic acid (hydroxy (methyl) benzoic acid), o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid (2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoic acid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), α-resorcylic acid (3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoic acid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid (3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxy Benzoic acid), 2,4-dimethoxybenzoic acid, orcerinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoic acid), syric acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid (2,4,5-trimethoxybenzoic acid), mandelic acid (hydroxy (phenyl) acetic acid), vanylmandelic acid (hydroxy (4-hydroxy-3-methoxyphenyl) acetic acid), homoanis acid ((4-methoxyphenyl) acetic acid), homogentisic acid ((2,5-dihydroxyphenyl) acetic acid), homoprotocatechuic acid ((3,4-dihydroxyphenyl) acetic acid), homovanillic acid ((4-hydroxy-3-methoxyphenyl) acetic acid), homoisovanillic acid ((3-hydroxy-4-methoxyphenyl) acetic acid), homoveratric acid ((3,4-dimethoxyphenyl) acetic acid), o-homoveratric acid ((2,3-dimethoxyphenyl) acetic acid), homophthalic acid (2-(carboxymethyl) benzoic acid), homoisophthalic acid (3-(carboxymethyl) benzoic acid), homoterephthalic acid (4-(carboxymethyl) benzoic acid), phthalic acid (2-(carboxycarbonyl) benzoic acid), isophthalic acid (3-(carboxycarbonyl) benzoic acid), terephthalic acid (4-(carboxycarbonyl) benzoic acid), atorolactic acid (2-hydroxy-2-phenylpropanoic acid), tropa acid (3-hydroxy-2-phenylpropanoic acid), melilotric acid (3-(2-hydroxyphenyl) propanoic acid), phloretic acid (3-(4-hydroxyphenyl) propanoic acid), hydrocaffeic acid (3-(3,4-dihydroxyphenyl), propanoic acid), hydroferulic acid (3-(4-hydroxy-3-methoxyphenyl) propanoic acid), hydroisofluric acid (3-(3-hydroxy-4-methoxyphenyl) propanoic acid), p-coumaric acid (3-(4-hydroxyphenyl) acrylic acid), humic acid (3-(2,4-dihydroxyphenyl) acrylic acid), caffeic acid (3-(3,4-dihydroxyphenyl) acrylic acid), ferulic acid (3-(4-hydroxy-3-methoxyphenyl) acrylic acid), isoferuric acid (3-(3-hydroxy-4-methoxyphenyl) acrylic acid), sinapinic acid (3-(4-hydroxy-3,5-dimethoxyphenyl) acrylic acid), and the like.

Examples of the aromatic carboxylic acid having a benzene ring substituted with a halogen atom include carboxylic acids, in each of which at least one hydrogen atom of a benzoic acid is substituted with a fluoro group, such as fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid, tetrafluorobenzoic acid, and pentafluorobenzoic acid; carboxylic acids, in each of which at least one hydrogen atom of a benzoic acid is substituted with a chloro group, such as chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, tetrachlorobenzoic acid, and pentachlorobenzoic acid; carboxylic acids, in each of which at least one hydrogen atom of a benzoic acid is substituted with a bromo group, such as bromobenzoic acid, dibromobenzoic acid, tribromobenzoic acid, tetrabromobenzoic acid, and pentabromobenzoic acid; and carboxylic acids, in each of which at least one hydrogen atom of a benzoic acid is substituted with an iodo group, such as iodobenzoic acid, diiodobenzoic acid, triiodobenzoic acid, tetraiodobenzoic acid, and pentaiodobenzoic acid.

The aromatic carboxylic acid having a benzene ring is preferably a compound represent by the following chemical formula (4):

(in the chemical formula (4), R³¹-R³⁵ each independently represent a hydrogen atom, an alkyl group having 1-4 carbon atoms, an aryl group having 6-10 carbon atoms, an amino group that may be substituted (—NR^(a)R^(b): R^(a) and R^(b) each independently represent a hydrogen atom, or an alkyl group having 1-4 carbon atoms, or a phenyl group), a hydroxy group, a halogen atom, or an acetoxy group).

Specific examples of the aromatic carboxylic acid having a condensed benzene ring to which a carboxy group is directly bonded include 1-naphthalenecarboxylic acid, 2-naphthalenecarboxylic acid, 1-anthracenecarboxylic acid, 2-anthracenecarboxylic acid, 9-anthracenecarboxylic acid, enanthrene carboxylic acid, pyrene carboxylic acid, and the like. Specific examples of the aromatic carboxylic acid having a condensed benzene ring to which a carboxy group is bonded via an alkylene group include naphthylacetic acid, naphthylpropionic acid, and the like.

Examples of the carboxylic acid having a condensed benzene ring substituted with an alkyl group, an aryl group, an amino group, a hydroxy group, an alkoxyl group, or an oxo group include 6-amino-2-naphthalenecarboxylic acid, 1,4-dihydroxy-2-naphthalenecarboxylic acid, 3,5-dihydroxy-2-naphthalenecarboxylic acid, 3,7-dihydroxy-2-naphthalenecarboxylic acid, 3-hydroxy-2-anthracene carboxylic acid, 9,10-dihydro-9,10-dioxo-1-anthracenecarboxylic acid, and the like.

Examples of the carboxylic acid having a condensed benzene ring substituted with a halogen atom include fluoronaphthalenecarboxylic acid, chloronaphthalenecarboxylic acid, bromonaphthalenecarboxylic acid, fluoroanthracene carboxylic acid, chloroanthracene carboxylic acid, bromoanthracene carboxylic acid, and the like.

Preferable examples of the aromatic carboxylic acid having a condensed benzene ring include a derivative having a naphthalenecarboxylic acid and/or a naphthalenecarboxylic acid structure in which one carboxy group is directly bonded to naphthalene, and a derivative having an anthracene carboxylic acid and/or an anthracene carboxylic acid structure in which one carboxy group is directly bonded to anthracene. The derivative having a naphthalenecarboxylic acid and/or a naphthalenecarboxylic acid structure is preferably a compound represented the following chemical formula (5-1) or (5-2). The derivative having an anthracene carboxylic acid and/or an anthracene carboxylic acid structure is preferably a compound represented the following chemical formula (6-1), (6-2) or (6-3).

In the chemical formulas (5-1) and (5-2), R⁴¹-R⁴⁷ each independently represent a hydrogen atom, an alkyl group having 1-4 carbon atoms, an aryl group having 6-10 carbon atoms, an amino group that may be substituted (—NR^(a)R^(b): R^(a) and R^(b) each independently represent a hydrogen atom, or an alkyl group having 1-4 carbon atoms, or a phenyl group), a hydroxy group, a halogen atom, or an acetoxy group.

In the chemical formulas (6-1), (6-2) and (6-3), R⁵¹-R⁵⁹ each independently represent a hydrogen atom, an alkyl group having 1-4 carbon atoms, an aryl group having 6-10 carbon atoms, an amino group that may be substituted (—NR^(a)R^(b): R^(a) and R^(b) each independently represent a hydrogen atom, or an alkyl group having 1-4 carbon atoms, or a phenyl group), a hydroxy group, a halogen atom, or an acetoxy group.

Specific examples of the aromatic carboxylic acid having an aromatic heterocycle to which a carboxy group is directly bonded include carboxylic acids each having a five-membered heterocyclic ring such as pyrrole carboxylic acid, furancarboxylic acid, thiophenecarboxylic acid (2-thenoyl acid), imidazole carboxylic acid, pyrazole carboxylic acid, oxazole carboxylic acid, and thiazole carboxylic acid; carboxylic acids having a six-membered heterocyclic ring such as pyridine carboxylic acid, pyrazinecarboxylic acid, pyridazinecarboxylic acid, pyrimidinecarboxylic acid, triazinecarboxylic acid, and tetrazinecarboxylic acid; carboxylic acids each having a condensed heterocyclic ring such as indole carboxylic acid, isoindole carboxylic acid, benzimidazole carboxylic acid, quinoline carboxylic acid, isoquinoline carboxylic acid, quinoxaline carboxylic acid, cinnoline carboxylic acid, quinazoline carboxylic acid, benzofuran carboxylic acid, benzothiophenecarboxylic acid, and benzothiazole carboxylic acid; and salts thereof. Among these, those having one hetero atom as a constituent atom of an aromatic heterocycle and having a carboxy group bonded at a position 2 or 3 with respect to the hetero atom on the heterocyclic ring are preferable, and those having one hetero atom as a constituent atom of an aromatic heterocycle and having a carboxy group bonded at a position 2 with respect to the hetero atom on the heterocyclic ring are more preferable.

The aromatic carboxylic acid having an aromatic heterocycle to which a carboxy group is directly bonded is preferably a compound represent by the following chemical formula (7) or (8):

(in the chemical formula (7), R⁶¹-R⁶⁴ each independently represent a hydrogen atom, a carboxy group, a halogen atom, a hydroxy group, a mercapto group, an alkyl group, an aryl group, an aralkyl group, an alkylaryl group, an alkoxyl group, an amino group that may be substituted, a cyano group, and a thiocarboxy group; however, at least one of R⁶¹-R⁶⁴ is a carboxy group)

(in the chemical formula (8), R⁷¹-R⁷⁴ each independently represent a hydrogen atom, a carboxy group, a halogen atom, a hydroxy group, a mercapto group, an alkyl group, an aryl group, an aralkyl group, an alkylaryl group, an alkoxyl group, an amino group that may be substituted, a cyano group, and a thiocarboxy group; however, at least one of R⁷¹-R⁷⁴ is a carboxy group).

Examples of the compound represented by the above chemical formula (7) include 2-furancarboxylic acid, 3-furancarboxylic acid, 5-chlorofuran-2-carboxylic acid, 5-bromofuran-2-carboxylic acid, 5-iodofuran-2-carboxylic acid, 4,5-dibromo-2-furancarboxylic acid, 3,5-dibromo-2-furancarboxylic acid, 3-methyl-2-furancarboxylic acid, 2-methyl-3-furancarboxylic acid, 5-methyl-2-furancarboxylic acid, 2,4-dimethyl-3-furancarboxylic acid, 2,5-dimethyl-3-furancarboxylic acid, 5-phenyl-2-furancarboxylic acid, 5-(hydroxymethyl) furan-2-carboxylic acid, 5-benzyl-3-(hydroxymethyl)-2-furancarboxylic acid, 5-methoxymethyl-2-furancarboxylic acid, 2-ethoxymethyl-5-methyl-3-furancarboxylic acid, 5-(ethoxymethyl)-2-methyl-3-furancarboxylic acid, 5-Aminofuran-2-carboxylic acid, and the like.

Examples of the compound represented by the above chemical formula (8) include 2-thenoyl acid, 3-thenoyl acid, 5-chlorothiophene-2-carboxylic acid, 5-bromothiophene-2-carboxylic acid, 5-iodothiophene-2-carboxylic acid, 3,5-dibromo-2-thiophenecarboxylic acid, 2,4,5-tribromo-3-thiophenecarboxylic acid, 3-hydroxy-2-thiophenecarboxylic acid, 4-hydroxy-2-thiophenecarboxylic acid, 3-methylthiophene-2-carboxylic acid, 4-methylthiophene-2-carboxylic acid, 5-methyl-3-thiophenecarboxylic acid, 5-methyl-2-thiophenecarboxylic acid, 5-phenylthiophene-2-carboxylic acid, 5-benzyl-2-thiophenecarboxylic acid, 4-benzyl-2-thiophenecarboxylic acid, 3-benzyl-2-thiophenecarboxylic acid, 3-methoxythiophene-2-carboxylic acid, 5-(methoxymethyl)-2-thiophenecarboxylic acid, 5-amino-2-thiophenecarboxylic acid, and the like.

Examples of the metal salt of the aromatic carboxylic acid include metal salts of the aromatic carboxylic acid having a benzene ring, the aromatic carboxylic acid having a condensed benzene ring, and the aromatic carboxylic acid having an aromatic heterocycle. Examples of the metal ion of the metal salt of the aromatic carboxylic acid include monovalent metal ions such as sodium, potassium, lithium and silver; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel and manganese; trivalent metal ions such as aluminum and iron; and other ions such as tin, zirconium, and titanium. A cation component of each of the carboxylic acid salts is preferably a zinc ion. These cation components may each be independently used or two or more of these cation components may be used in combination.

A blending amount of the aromatic carboxylic acid and/or the metal salt thereof, with respect to 100 parts by mass of the base rubber, is preferably 1.0 part by mass or more, more preferably 2.0 parts by mass or more, even more preferably 2.5 parts by mass or more, and particularly preferably 3.0 parts by mass or more, and is preferably 40 parts by mass or less, more preferably 35 parts by mass or less, even more preferably 30 parts by mass or less, and particularly preferably 10 parts by mass or less. When the blending amount of the aromatic carboxylic acid and/or the metal salt thereof is 1.0 part by mass or more, an effect of adding the aromatic carboxylic acid and/or the metal salt thereof is increased, and a degree of gradational hardness of the spherical core is further increased. When the blending amount of the aromatic carboxylic acid and/or the metal salt thereof is 40 parts by mass or less, a decrease in hardness of the entire resulting spherical core is suppressed, and a satisfactory resilience is obtained.

(f) Metal Compound

When the rubber composition in an embodiment of the present invention contains only the α,β-unsaturated carboxylic acid having 3-8 carbon atoms as the co-cross-linking agent, the rubber composition further contains a metal compound as a component. The metal compound is not particularly limited as long as the metal compound can neutralize the α,β-unsaturated carboxylic acid having 3-8 carbon atoms in the rubber composition. Examples of the metal compound include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. The metal compound is preferably a divalent metal compound, and more preferably a zinc compound. This is because the divalent metal compound reacts with the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby forming metal cross-links. Further, by using a zinc compound, a golf ball highly resilient is obtained. The metal compounds may each be independently used, or two or more of the metal compounds may be used in combination. A content of the metal compound may be appropriately determined according to a desired neutralization degree.

(g) Organic Sulfur Compound

The rubber composition in an embodiment of the present invention preferably further contains an organic sulfur compound. When the rubber composition contains the organic sulfur compound, the resilience of the spherical core is further improved. The organic sulfur compound is not particularly limited as long as the organic sulfur compound has a sulfur atom in its molecule. Examples of the organic sulfur compound include an organic compound having a thiol group (—SH) or a polysulfide bond having 2-4 sulfur atoms (—S—S—, —S—S—S— or —S—S—S—S—), and a metal salt thereof (—SM, —S-M-S—, —S-M-S—S—, —SS-M-S—S—, —S-M-S—S—S—, or the like, where M is a metal atom). Examples of the metal salt include salts of monovalent metals such as sodium, lithium, potassium, copper (I), and silver (I); and salts of divalent metals such as zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), and tin (II). Further, (g) the organic sulfur compound may be any one of an aliphatic compound (such as aliphatic thiol, aliphatic thiocarboxylic acid, aliphatic dithiocarboxylic acid, or aliphatic polysulfide), a heterocyclic compound, an alicyclic compound (such as alicyclic thiol, alicyclic thiocarboxylic acid, alicyclic dithiocarboxylic acid, or alicyclic polysulfide), and an aromatic compound.

Examples of the organic sulfur compound include thiols (thiophenols and thionaphthols), polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, and the like.

Examples of the thiols include thiophenols and thionaphthols. Examples of the thiophenols include thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, and pentachlorothiophenol; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; and metal salts thereof. As the metal salt, a zinc salt is preferable.

Examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and metal salts thereof. Preferable examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, and metal salts thereof. The metal salt is preferably a divalent metal salt, and more preferably a zinc salt. Specific examples of the metal salt include a zinc salt of 1-thionaphthol and a zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond. Examples of the polysulfides are organic sulfur compounds include disulfides, trisulfides, and tetrasulfides. As the polysulfides, diphenylpolysulfides are preferable.

Examples of the diphenylpolysulfides include, in addition to diphenyldisulfide, diphenyldisulfides substituted with a halogen group, such as bis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide, and bis(pentaiodophenyl)disulfide; and diphenyldisulfides substituted with an alkyl group, such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, and bis(penta-t-butylphenyl)disulfide; and the like.

Examples of the thiurams include thiuram monosulfides such as tetramethylthiuram monosulfide; thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetrabutylthiuram disulfide; and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylic acids include a naphthalene thiocarboxylic acid. Examples of the dithiocarboxylic acids include a naphthalene dithiocarboxylic acid. Examples of the sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

Preferable examples of the organic sulfur compound include thiophenols and/or metal salt thereof, thionaphthols and/or metal salt thereof, diphenyldisulfides, and thiuramdisulfides. More preferable examples of the organic sulfur compound include 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyldisulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, and bis(pentabromophenyl) disulfide.

These organic sulfur compounds can each be independently used, or two or more of these organic sulfur compounds can be used in combination.

A content of the organic sulfur compound, with respect to 100 parts by mass of the base rubber, is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.2 parts by mass or more, and is preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.0 parts by mass or less. When the content of the organic sulfur compound is less than 0.05 parts by mass, there is a risk that an effect of adding the organic sulfur compound is not achieved, and the resilience of the golf ball is not improved. Further, when the content of the organic sulfur compound exceeds 5.0 parts by mass, there is a risk that a compression deformation amount of the resulting golf ball may increase and the resilience of the golf ball may decrease.

Rubber Composition

A ratio (component (d)/component (b)) of a total number of moles of carbon-carbon double bonds of the unsaturated fatty acid and/or the metal salt thereof to a total number of moles of carbon-carbon double bonds of the α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or the metal salt thereof is preferably 0.01 or more, more preferably 0.03 or more, and even more preferably 0.06 or more, and is preferably 0.20 or less, more preferably 0.18 or less, and even more preferably 0.16 or less. When the ratio (component (d)/component (b)) is 0.01 or more, the addition reaction between the component (d) and the component (b) more easily occurs, and the resulting spherical core becomes highly resilient. On the other hand, when the ratio (component (d)/component (b)) is 0.20 or less, the durability of the golf ball can be maintained without changing a compression deformation amount of the spherical core.

A ratio ((component (d)+component (e))/component (b)) of a total number of moles of carboxy groups (—COOH) and carboxylate groups (—COO⁻) of the component (d) and the component (e) to a total number of moles of carboxy groups and carboxylate groups of the component (b) is preferably 0.03 or more, more preferably 0.04 or more, and even more preferably 0.05 or more, and is preferably 4 or less, more preferably 3 or less, and even more preferably 2 or less.

A ratio (component (d))/component (e)) of a total number of moles of carboxy groups and carboxylate groups of the component (d) to a total number of moles of carboxy groups and carboxylate groups of the component (e) is preferably 0.01 or more, more preferably 0.025 or more, and even more preferably 0.035 or more, and is preferably 18 or less, more preferably 14 or less, and even more preferably 10 or less. When the ratio (component (d)/component (e)) is 0.01 or more, a spherical core having adequate flexibility is obtained, and when the ratio (component (d)/component (e)) is 18 or less, the spherical core has a satisfactory resilience.

A total number of moles of carboxy groups and carboxylate groups of the component (d) in the rubber composition, with respect to 100 parts by mass of the base rubber, is preferably 0.003 or more, more preferably 0.006 or more, and even more preferably 0.009 or more, and is preferably 0.155 or less, more preferably 0.130 or less, and even more preferably 0.110 or less. When the total number of moles of carboxy groups and carboxylate groups of the component (d) is within the above range, the resilience of the resulting spherical core is further improved.

A total number of moles of carboxy groups and carboxylate groups of the component (e) in the rubber composition, with respect to 100 parts by mass of the base rubber, is preferably 0.009 or more, more preferably 0.011 or more, and even more preferably 0.013 or more, and is preferably 0.33 or less, more preferably 0.29 or less, and more preferably 0.25 or less. When the total number of moles of carboxy groups and carboxylate groups of the component (e) is with the above range, the gold ball formed using the resulting spherical core has low spin on a driver shot.

A total number of moles of carboxy groups and carboxylate groups of the component (d) and carboxy groups and carboxylate groups of the component (e) in the rubber composition, with respect to 100 parts by mass of the base rubber, is preferably 0.012 or more, more preferably 0.017 or more, and even more preferably 0.022 or more, and is preferably 0.49 or less, more preferably 0.42 or less, and more preferably 0.36 or less. When the total number of moles of the carboxy groups and the carboxylate groups is with the above range, the flight distance on a driver shot of the gold ball formed using the resulting spherical core is further improved.

A total blending amount of the unsaturated fatty acid and/or metal salt thereof and the saturated fatty acid and/or metal salt thereof in rubber composition, with respect to 100 parts by mass of the base rubber, is preferably 2 parts by mass or more, more preferably 3 parts by mass or more, even more preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, even more preferably 40 parts by mass or less, particularly preferably 30 parts by mass or less, and most preferably 25 parts by mass or less. When the total content of the component (d) and the component (e) is within the above range, the flight distance on a driver shot of the golf ball formed using the resulting spherical core is further improved.

In the rubber composition, a neutralization degree of carboxy groups (alkali equivalents of metal ions when acid equivalents of carboxy groups and carboxylate groups in the rubber composition is 100 mol %) is preferably 100 mole % or more, more preferably 105 mole % or more, even more preferably 108 mole % or more, and particularly preferably 110 mole % or more, and and is preferably 200 mole % or less, more preferably 180 mole % or less, even more preferably 170 mole % or less, and particularly preferably 160 mole % or less. When the neutralization degree is 100 mole % or more, the durability of the golf ball can be maintained without changing the compression deformation amount of the core. On the other hand, when the neutralization degree is 200 mole % or less, the resulting spherical core does not become excessively soft, and the high resilience of the golf ball is not impaired. The neutralization degree of the spherical core is defined by the following mathematical formula.

$\begin{matrix} {\left. {{{Neutralization}\mspace{14mu} {degree}\mspace{11mu} \left( {{mole}\mspace{14mu} \%} \right)} = \frac{\left( {\; \begin{matrix} {\sum\left( {{mole}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {caution}} \right.} \\ {{component} \times} \\ \left. {{valence}\mspace{14mu} {of}\mspace{14mu} {cation}\mspace{14mu} {component}} \right) \end{matrix}\mspace{11mu}} \right)}{\left( {\begin{matrix} {\sum\left( {{mole}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {anion}} \right.} \\ {{component} \times} \\ \left. {{valence}\mspace{14mu} {of}\mspace{14mu} {anion}\mspace{14mu} {component}} \right) \end{matrix}\mspace{14mu}} \right)}} \right\} \times 100} & {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} 1} \end{matrix}$

In the mathematical formula (1), Σ((number of moles of cation component)×(valence of cation component)) is a sum of a product of a number of moles of metal ions and a valence of the metal ions of the component (b), a product of a number of moles of metal ions and a valence of the metal ions of the component (d), a product of a number of moles of metal ions and a valence of the metal ions of the component (e), and a product of a number of moles of metal ions and a valence of the metal ions of the component (f). Σ((number of moles of anion component)×(valence of anion component)) is a sum of a total number of moles of carboxy groups and carboxylate groups of the component (b), a total number of moles of carboxy groups and carboxylate groups of the component (d), and a total number of moles of carboxy groups and carboxylate groups of the component (e).

The rubber composition in an embodiment of the present invention may further contain additives such as a pigment, filler for adjusting a weight or the like, an anti-aging agent, a peptizing agent, and a softening agent, when necessary. Further, the rubber composition for the core may also contain rubber powder obtained by pulverizing cores of golf balls and edge materials generated during production of cores.

Examples of the pigment blended into the rubber composition can include a white pigment, a blue pigment, a purple pigment and the like. As the white pigment, titanium oxide is preferably used. A type of the titanium oxide is not particularly limited, but a rutile type is preferably used because of high opacity. Further, a content of the titanium oxide, with respect to 100 parts by mass of the base rubber, is preferably 0.5 parts by mass or more, and more preferably 2 parts by mass or more, and is preferably 8 parts by mass or less, and more preferably 5 parts by mass or less.

It is also preferable that the rubber composition contain both a white pigment and a blue pigment. The blue pigment is blended in order to make a white color vivid. Examples of blue pigments include ultramarine blue, cobalt blue, phthalocyanine blue, and the like. Further, examples of the purple pigment include anthraquinone violet, dioxazine violet, methyl violet, and the like.

The filler blended in the rubber composition is mainly used as a weight adjusting agent for adjusting a weight of the golf ball obtained as a final product. The filler may be blended when necessary. Examples of the filler include inorganic filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, molybdenum powder, and the like. Zinc oxide is particularly preferably used as the filler.

It is believed that the zinc oxide functions as a vulcanization agent and increases the hardness of the entire spherical core. A content of the filler, with respect to 100 parts by mass of the base rubber, is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, and is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less. When the content of the filler is less than 0.5 parts by mass, weight adjustment becomes difficult. When the content of the filler exceeds 30 parts by mass, a weight ratio of the rubber component is reduced and the resilience tends to decrease.

A content of the anti-aging agent, with respect to 100 parts by mass of the base rubber, is preferably 0.1 parts by mass or more and 1 part by mass or less. Further, a content of the peptizing agent, with respect to 100 parts by mass of the base rubber, is preferably 0.1 parts by mass or more and 5 parts by mass or less.

Spherical Core

A diameter of the spherical core is preferably 34.8 mm or more, more preferably 36.8 mm or more, and even more preferably 38.8 mm or more, and is preferably 42.2 mm or less, more preferably 41.8 mm or less, even more preferably 41.2 mm or less, and most preferably 40.8 mm or less. When the diameter of the spherical core is 34.8 mm or more, a thickness of the cover does not become excessively large and the resilience is further improved. On the other hand, when the diameter of the spherical core is 42.2 mm or less, the thickness of the cover does not become excessively small and the function of the cover is further enhanced.

When the diameter of the spherical core is in a range of 34.8 mm-42.2 mm, a compression deformation amount of the spherical core (an amount by which the spherical core shrinks along a compression direction) when a load is applied and is increased from an initial load of 98 N to a final load of 1275 N is preferably 1.90 mm or more, more preferably 2.00 mm or more, and even more preferably 2.10 mm or more, and is preferably 4.00 mm or less, more preferably 3.90 mm or less, and even more preferably 3.80 mm or less. When the compression deformation amount is 1.90 mm or more, the shot feeling of the golf ball is further improved. When the compression deformation amount is 4.00 mm or less, the resilience of the golf ball is further improved.

A hardness difference (Hs−Ho) between a surface hardness (Hs) and a center hardness (Ho) of the spherical core, in Shore C hardness, is preferably 1 or more, more preferably 5 or more, even more preferably 7 or more, and particularly preferably 10 or more, and is preferably 90 or less, more preferably 80 or less, and even more preferably 70 or less. When the hardness difference is large, a golf ball having a high launch angle, a low spin and a long flight distance is obtained.

The center hardness (Ho) of the spherical core, in Shore C hardness, is preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more. When the center hardness (Ho) of the spherical core is 10 or more in Shore C hardness, the spherical core does not become excessively soft, and a satisfactory resilience is obtained. Further, the center hardness (Ho) of the spherical core, in Shore C hardness, is preferably 90 or less, more preferably 80 or less, and even more preferably 70 or less. When the center hardness (Ho) is 90 or less in Shore C hardness, the spherical core does not become excessively hard, and a satisfactory shot feeling of the golf ball is obtained.

The surface hardness (Hs) of the spherical core, in Shore C hardness, is preferably 30 or more, more preferably 40 or more, and even more preferably 50 or more, and is preferably 100 or less, more preferably 90 or less, and even more preferably 80 or less. When the surface hardness of the spherical core is 30 or more in Shore C hardness, the spherical core does not become excessively soft, and a satisfactory resilience is obtained. When the surface hardness of the spherical core is 100 or less in Shore C hardness, the spherical core does not become excessively hard, and a satisfactory shot feeling of the golf ball is obtained.

Cover

The cover of a golf ball according to an embodiment of the present invention is formed from a cover composition containing a resin component. Examples of the resin component include an ionomer resin; a thermoplastic polyurethane elastomer commercially available under a trade name of “Elastollan (registered trademark)” from BASF Japan Ltd.; a thermoplastic polyamide elastomer commercially available under a trade name of “Pebax (registered trademark)” from Arkema Corporation; a thermoplastic polyester elastomer commercially available under a trade name of “Hytrel (registered trademark)” from Du Pont-Toray Co., Ltd.; and a thermoplastic styrene elastomer commercially available under a trade name of “Rabalon (registered trademark)” from Mitsubishi Chemical Corporation; and the like.

Examples of the ionomer resin include a product obtained by neutralizing at least a part of carboxyl groups in a binary copolymer composed of an olefin and an α,β-unsaturated carboxylic acid having 3-8 carbon atoms with metal ions; a product obtained by neutralizing at least a part of carboxyl groups in a ternary copolymer composed of an olefin, an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and an α,β-unsaturated carboxylic acid ester with metal ions; and a mixture of those. As the olefin, an olefin having 2-8 carbon atoms is preferable. Examples of the olefin include ethylene, propylene, butene, pentene, hexene, heptene, octene, and the like. The ethylene is particularly preferable. Examples of the α,β-unsaturated carboxylic acid having 3-8 carbon atoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, and the like. Among these, acrylic acid and methacrylic acid are particularly preferred. Further, as the α,β-unsaturated carboxylic acid ester, for example, methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid or the like are used. In particular, acrylic acid ester and methacrylic acid ester are preferred. Among these, as the ionomer resin, a metal ion-neutralized product of a binary copolymer composed of ethylene and (meth)acrylic acid, and a metal ion-neutralized product of a ternary copolymer composed of ethylene, (meth)acrylic acid and (meth)acrylic acid ester are preferable.

Specific examples of the ionomer resin include, in trade names, “Himilan (registered trademark) (for example, Himilan 1555 (Na), 1557 (Zn), 1605 (Na), 1706 (Zn), 1707 (Na), AM3711 (Mg), and the like; and ternary copolymerized ionomer resins such as Himilan 1856 (Na), and 1855 (Zn))” available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples of ionomer resins commercially available from DuPont include “Surlyn (registered trademark) (for example, Surlyn 8945 (Na), 9945 (Zn), 8140 (Na), 8150 (Na), 9120 (Zn), 9150 (Zn), 6910 (Mg), 6120 (Mg), 7930 (Li), 7940 (Li), AD8546 (Li), and the like; and ternary copolymerized ionomer resins such as Surlyn 8120 (Na), 8320 (Na), 9320 (Zn), 6320 (Mg), HPF 1000 (Mg), and HPF 2000 (Mg)).”

Further, examples of ionomer resins commercially available from Exxon Mobil Chemical Corporation include “Iotek (registered trademark) (for example, Iotek 8000 (Na), 8030 (Na), 7010 (Zn), 7030 (Zn), and the like; and ternary copolymerized ionomer resins such as Iotek 7510 (Zn), and 7520 (Zn)).”

Na, Zn, Li, Mg and the like described in the parentheses after the trade names of the ionomer resins respectively indicate metal types of the metal ions for neutralizing the ionomer resins. These ionomer resins may each be independently used, or two or more of these ionomer resins may be used in combination.

The cover composition that forms the cover of a golf ball according to an embodiment of the present invention preferably contains, as a resin component, a thermoplastic polyurethane elastomer or an ionomer resin. When the ionomer resin s used, it is also preferable to use a thermoplastic styrene elastomer in combination. A content rate of the polyurethane or ionomer resin in the resin component of the cover composition is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more.

In addition to the above-described resin component, the cover composition may further contain a pigment component such as a white pigment (such as titanium oxide), a blue pigment and a red pigment; a weight adjusting agent such as zinc oxide, calcium carbonate and barium sulfate; a dispersant; an anti-aging agent; an ultraviolet absorber; a light stabilizer; a fluorescent material or a fluorescent brightener; and the like, as long as they do not impair the performance of the cover.

A content of the white pigment (such as titanium oxide), with respect to 100 parts by mass of the resin component that forms the cover, is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less. When the content of the white pigment is 0.5 parts by mass or more, it is possible to impart opacity to the cover. Further, when the content of the white pigment exceeds 10 parts by mass, durability of the resulting cover may deteriorate.

It is preferable to appropriately set slab hardness of the cover composition in accordance with desired performance of the golf ball. For example, for a distance-type golf ball that emphasizes a flight distance, a slab hardness of the cover composition in Shore D hardness is preferably 50 or more, more preferably 55 or more, and even more preferably 60 or more, and is preferably 80 or less, more preferably 70 or less, and even more preferably 68 or less. When the slab hardness of the cover composition is 50 or more, a golf ball having a high launch angle and a low spin on a driver shot and on an iron shot is obtained, and the flight distance is improved. Further, when the slab hardness of the cover composition is 80 or less, a golf ball excellent in durability is obtained. Further, for a spin-type golf ball that emphasizes controllability, the slab hardness of the cover composition in Shore D hardness is preferably less than 50, and is preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more. When the slab hardness of the cover composition is less than 50 in Shore D hardness, a high flight distance on a driver shot can be achieved by a core according to an embodiment of the present invention, and a golf ball that has an increased amount of spin on an approach shot and easily stops on the green is obtained. Further, when the slab hardness of the cover composition is 20 or more, abrasion resistance is improved. When multiple cover layers are provided, slab hardnesses of cover compositions that respectively for the cover layers may be identical to or different from each other as long as the slab hardnesses of the layers are within the above range.

Examples of a method for molding the cover of a golf ball according to an embodiment of the present invention include a method in which hollow shells are molded from the cover composition and the core is covered with multiple hollow shells and the resulting object is subjected to compression molding (preferably, a method in which hollow half shells are molded from the cover composition and the code is covered with two half shells and the resulting object is subjected to compression molding), and a method in which the cover composition is directly injection-molded onto the core.

When the cover is molded using a compression molding method, molding of half shells may be performed using either a compression molding method or an injection molding method. However, the compression molding method is preferable. Conditions for compression molding the cover composition into half shells can include, for example, a pressure of 1 MPa or more and 20 MPa or less and a molding temperature of −20° C. or more and 70° C. or less relative to a flow beginning temperature of the cover composition. By performing the molding under the above conditions, half shells having a uniform thickness can be molded. An example of a method for molding the cover using half shells is a method in which the core is covered with two half shells and the resulting object is subjected to compression molding. Conditions for molding the cover by compression molding half shells can include, for example, a molding pressure of 0.5 MPa or more and 25 MPa or less and a molding temperature of −20° C. or more and 70° C. or less relative to a flow beginning temperature of the cover composition. By performing the molding under the above conditions, a golf ball cover having a uniform thickness can be molded.

When the cover composition is injection molded into the cover, it is possible that the cover composition extruded in a pellet form beforehand is used for the injection molding, or the cover materials such as the base resin component and the pigments are dry blended and the blended material is directly injection molded. As upper and lower molds for cover molding, it is preferable to use molds having hemispherical cavities and pimples, in which some of the pimples also serve as retractable hold pins. When the cover is molded by injection molding, the hold pins are protruded, the core is put in and is held by the hold pins, and thereafter, the cover composition is charged and then cooled, and thereby, the cover is molded. For example, the molding of the cover may be performed as follows: the cover composition heated to a temperature in a range of 200° C.-250° C. is charged in 0.5-5 seconds into a mold held under a pressure in a range of 9 MPa-15 MPa, and then is cooled for 10 seconds-60 seconds, and then the mold is opened.

When the cover is molded, concave portions called “dimples” are usually formed on a surface of the cover. A total number of dimples formed on the cover is preferably 200 or more and 500 or less. When the total number of dimples is less than 200, a dimple effect is unlikely to be obtained. On the other hand, when the total number of dimples exceeds 500, a size of each of the dimples becomes smaller and the dimple effect is unlikely to be obtained. A shape (shape in a plan view) of each of the formed dimples is not particularly limited. Examples of shapes of the dimples include a circle; a polygonal shape such as a substantially triangular shape, a substantially quadrangular shape, a substantially pentagonal shape, and a substantially hexagonal shape; and other irregular shape. These shapes of the dimples may each be independently used, or two or more of these shapes may be used in combination.

The thickness of the cover is preferably 4.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less. When the thickness of the cover is 4.0 mm or less, the resilience and shot feeling of the resulting golf ball are further improved. The thickness of the cover is preferably 0.3 mm or more, more preferably 0.5 mm or more, even more preferably 0.8 mm or more, and particularly preferably 1.0 mm or more. When the thickness of the cover is less than 0.3 mm, the durability and wear resistance of the cover may deteriorate. When multiple cover layers are provided, a total thickness of the multiple cover layers is preferable within the above range.

A golf ball body having the cover molded thereon is ejected from the mold and, when necessary, is preferably subjected to surface treatments such as deburring, cleaning and sandblast. Further, when desired, a coating film or a mark can be formed. A thickness of the coating film is not particularly limited, but is preferably 5 μm or more, and more preferably 7 μm or more, and is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. When the thickness of the coating film is less than 5 μm, the coating film is easy to wear off due to continued use of the golf ball. When the thickness of the coating film exceeds 50 μm, the dimple effect is decreased and the flight performance of the golf ball is decreased.

Golf Ball

A structure of a golf ball according to an embodiment of present invention is not particularly limited as long as the structure has a spherical core and a cover of one or more cover layers covering the spherical core. The spherical core preferably has a single-layer structure. Unlike a multi-layer structure, the spherical core of the single-layer structure does not have energy loss at an interface of the multi-layer structure when being hit, and the resilience of the spherical core is improved. Further, the cover has a structure of one or more layers, and may have a single-layer structure or a multi-layer structure of two or more layers. Examples of a golf ball according to an embodiment of the present invention include a two-piece golf ball that includes a spherical core and a single-layer cover arranged so as to cover the spherical core; a multi-piece golf ball (including a three-piece golf ball) that includes a spherical core and a cover of two or more cover layers arranged so as to cover the spherical core; and a thread-wound golf ball that includes a spherical core, a thread rubber layer provided around the spherical core, and a cover arranged so as to over the thread rubber layer. An embodiment of the present invention can be suitably applied to golf balls having any one of the above structures.

A golf ball according to an embodiment of the present invention preferably has a diameter ranging from 40 mm to 45 mm. From a point of view of satisfying standards of the US Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. From a point of view of air resistance suppression, the diameter is more preferably 44 mm or less, and particularly preferably 42.80 mm or less. Further, a golf ball according to an embodiment of the present invention preferably has a mass of 40 g or more and 50 g or less. From a point of view that a inertia can be obtained, the mass is more preferably 44 g or more, and particularly preferably 45.00 g or more. From a point of view of satisfying standards of the USGA, the mass is particularly preferably 45.93 g or less.

When a golf ball according to an embodiment of the present invention has a diameter in a range of 40 mm-45 mm, the compression deformation amount of the golf ball (shrinking amount of the golf ball along the compression direction) when a load is applied and is increased from an initial load of 98 N to a final load of 1275 N is preferably 2.0 mm or more, more preferably 2.2 mm or more, and even more preferably 2.4 mm or more, and is preferably 4.0 mm or less, more preferably 3.5 mm or less, and even more preferably 3.4 mm or less. When the compression deformation amount is 2.0 mm or more, the golf ball does not become excessively hard and a satisfactory shot feeling of the golf ball is obtained. On the other hand, when the compression deformation amount is 4.0 mm or less, the resilience of the golf ball is increased.

FIGURE is a partially cutaway cross-sectional view illustrating a golf ball according to an embodiment of the present invention. A golf ball 1 has a spherical core 2 and cover 3 covering the spherical core 2. A large number of dimples 31 are formed on a surface of the cover. Other portion than the dimples 31 of the surface of the cover 3 is a land 32. The golf ball 1 has a paint layer and a mark layer on an outer side of the cover 3. However, illustration of the paint layer and the mark layer is omitted.

EXAMPLES

In the following, the present invention is described in detail by way of examples. However, the present invention is not limited to the examples described below. Various changes and modifications without departing from the spirit of the present invention are included in the scope of the present invention.

Evaluation Methods

(1) Compression Deformation Amount (mm)

A deformation amount in a compression direction (a shrinking amount of the core or golf ball along the compression direction) when a load is applied to the core or golf ball and is increased from an initial load of 98 N to a final load of 1275 N was measured.

(2) Coefficient of Restitution

A metal cylindrical object having a mass of 198.4 g was caused to collide with each core or golf ball at a speed of 40 m/sec. The speeds of the cylindrical object and the core or golf ball before and after the collision were measured. Based on the speeds and the masses of these objects, a coefficient of restitution of each core or golf ball was calculated. Twelve measurements were conducted for each core or golf ball, and an average value thereof was adopted as the coefficient of restitution for the each core or golf ball. The coefficient of restitution is expressed as a difference from the coefficient of restitution of a golf ball No. 16.

(3) Core Hardness (Shore C Hardness)

A hardness measured at a surface of a core was adopted as a core surface hardness. Further, the core was cut into hemispheres, and hardness was measured as a center of a cut surface. The hardness was measured using an automatic hardness tester (Digitest II, commercially available from H. Barleys Company). As a detector, “Shore C” was used.

(4) Slab Hardness (Shore D Hardness)

Sheets each having a thickness of about 2 mm were produced by injection molding the cover composition, and were stored at 23° C. for two weeks. Three or more of these sheets were stacked on one another so as not to be affected by a measurement substrate or the like, and a hardness of the stack was measured using an automatic hardness tester (Digitest II, commercially available from H. Barleys Company). As a detector, “Shore D” was used.

(5) Flight Distance on Driver Shot

A W #1 driver provided with a metal head (XXIO S, loft angel: 11 degrees, commercially available from Dunlop Sports Ltd.) was attached to a swing robot M/C commercially available from Golf Laboratories, Inc. A golf ball was hit at a head speed of 40 m/sec, and a flight distance (a distance from a launch point to a stop point) was measured. The measurement was conducted twelve times for each golf ball, and an average value thereof was adopted as a measured value for the each golf ball. The flight distance of each golf ball was expressed as a difference from the flight distance of the golf ball No. 16 (flight distance difference=flight distance of each golf ball−flight distance of golf ball No. 16).

(6) Durability

A W #1 driver provided with a metal head (XXIO S, loft angel: 11 degrees, commercially available from Dunlop Sports Ltd.) was attached to a swing robot M/C commercially available from Golf Laboratories, Inc. A golf ball was hit repeatedly at a head speed of 45 m/sec, and a hitting number until a crack occurred was measured. Twelve measurements were conducted for each golf ball, and an average value thereof was adopted as the hitting number for the each golf ball. The hitting number of each golf ball was evaluated according to the following criteria by calculating a difference from the hitting number of the golf ball No. 16 (hitting number difference=hitting number of each golf ball−hitting number of golf ball golf ball No. 16).

Evaluation Criteria

O: Hitting number difference is 0 or more.

X: Hitting number difference is less than 0.

(1) Production of Core

Each of rubber compositions of formulations shown in Table 1-3 was kneaded using a kneading roll, and the kneaded material was heated and pressed in upper and lower molds, each having a hemispherical cavity, at 170° C. for 20 minutes, and thereby, a spherical core having a diameter of 39.8 mm was obtained. Barium sulfate was added in an appropriate amount such that the resulting golf ball had a mass of 45.4 g.

TABLE 1 Golf ball No. 1 2 3 4 5 6 7 Rubber Contents (a) BR730 100 100 100 100 100 100 100 composition (parts by (b), (f) ZN-DA90S 29 29 29 29 27 28 29 mass) (c) Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 0.8 (d) Oleic acid 5 10 15 20 — — — (C18:1) Zinc ricinoleate — — — — 11 — — (C18:1) Erucic acid — — — — — 12 — (C22:1) Millistoleic acid — — — — — — 8 (C14:1) (e) Benzoic acid 2 2 2 2 2 2 2 (g) Zinc oxide 5 10 15 15 5 5 5 (h) PBDS 0.63 0.60 0.60 0.60 0.60 0.60 0.60 Barium sulfate Appro- Appro- Appro- Appro- Appro- Appro- Appro- priate priate priate priate priate priate priate amount amount amount amount amount amount amount Number of carbon-carbon double 3.54 3.54 3.54 3.54 3.03 2.95 4.42 bonds of (d) component (mmol/g) Mole ratio ((d)/(b)) of total numbers 0.07 0.14 0.21 0.28 0.14 0.15 0.14 of carbon-carbon double bonds Mole ratio ((d + e)/(b)) of carboxy 0.14 0.21 0.28 0.35 0.21 0.21 0.21 groups and carboxylate groups Mole ratio ((d)/(e)) of carboxy 1.08 2.16 3.24 4.32 2.03 2.16 2.16 groups and carboxylate groups Total number of moles of carboxy 0.018 0.035 0.053 0.071 0.033 0.035 0.035 groups and carboxylate groups (d) Total number of moles of carboxy 0.016 0.016 0.016 0.016 0.016 0.016 0.016 groups and carboxylate groups (e) Total number of moles of carboxy 0.034 0.052 0.069 0.087 0.050 0.052 0.052 groups and carboxylate groups ((d) + (e)) Neutralization degree (mol %) 130 162 191 181 136 123 123 Spherical Compression deformation amount 3.8 3.9 3.7 3.8 3.8 3.8 3.8 core (mm) Coefficient of restitution 0.010 0.010 0.010 0.010 0.008 0.010 0.008 Center hardness (Ho) (Shore C) 45 45 45 47 46 45 45 Surface hardness (Hs) (Shore C) 75 72 69 66 72 72 72 Hardness difference (Hs − Ho) 30 27 24 19 26 27 27 (Shore C) Golf ball Compression deformation amount 3.3 3.4 3.2 3.3 3.3 3.3 3.3 (mm) Coefficient of restitution 0.010 0.010 0.010 0.010 0.008 0.010 0.008 Flight distance on driver shot (yd) 2.4 2.4 2.4 2.2 2.0 2.4 2.1 Durability O O O O O O O

TABLE 2 Golf ball No. 8 9 10 11 12 Rubber Contents (a) BR730 100 100 100 100 100 composition (parts by (b), (f) ZN-DA90S 27 28 28 31 28 mass) (c) Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 (d) Oleic acid (C18:1) 5 5 5 5 5 (e) Benzoic acid 3 — — — — Dimethylaminobenzoic — 2.8 — — — acid Anisic acid — — 5 — — Chlorobenzoic acid Acetoxybenzoic acid — — — 5 — 2,4-dimenthoxybenzoic — — — — 2.6 acid (g) Zinc oxide 5 5 5 5 5 (h) PBDS 0.60 0.60 0.60 0.60 0.60 Barium sulfate Appro- Appro- Appro- Appro- Appro- priate priate priate priate priate amount amount amount amount amount Number of carbon-carbon double 3.54 3.54 3.54 3.54 3.54 bonds of (d) component (mmol/g) Mole ratio ((d)/(b)) of total numbers 0.08 0.07 0.07 0.07 0.07 of carbon-carbon double bonds Mole ratio ((d + e)/(b)) of carboxy 0.15 0.15 0.20 0.17 0.13 groups and carboxylate groups Mole ratio ((d)/(e)) of carboxy 0.97 0.96 0.55 0.64 1.24 groups and carboxylate groups Total number of moles of carboxy 0.018 0.018 0.018 0.018 0.018 groups and carboxylate groups (d) Total number of moles of carboxy 0.018 0.018 0.032 0.028 0.014 groups and carboxylate groups (e) Total number of moles of carboxy 0.036 0.036 0.050 0.045 0.032 groups and carboxylate groups ((d) + (e)) Neutralization degree (mol %) 131 130 124 124 132 Spherical Compression deformation amount 4.0 4.0 4.0 3.9 4.0 core (mm) Coefficient of restitution 0.004 0.009 0.005 0.006 0.007 Center hardness (Ho) (Shore C) 45 45 44 45 45 Surface hardness (Hs) (Shore C) 75 75 75 77 75 Hardness difference (Hs − Ho) 30 30 31 32 30 (Shore C) Golf ball Compression deformation amount 3.5 3.5 3.5 3.4 3.5 (mm) Coefficient of restitution 0.004 0.009 0.005 0.006 0.007 Flight distance on driver shot (yd) 1.6 2.3 1.8 1.9 2.0 Durability O O O O O

TABLE 3 Golf ball No. 13 14 15 16 17 18 Rubber Contents (a) BR730 100 100 100 100 100 100 composition (parts by (b), (f) ZN-DA90S 28 29 27 28 28 28 mass) (c) Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 (d) Oleic acid (C18:1) 5 5 5 — 5 — (e) Benzoic acid — — — — — 2 2,4,5- 3 — — — — — trimethoxybenzoic acid 2,4,6- — 6.4 — — — — trichlorobenzoic acid 4-tert-butylbenzoic — — 5 — — — acid (g) Zinc oxide 5 5 5 5 4 5 (h) PBDS 0.60 0.60 0.60 0.63 0.63 0.63 Barium sulfate Appro- Appro- Appro- Appro- Appro- Appro- priate priate priate priate priate priate amount amount amount amount amount amount Number of carbon-carbon double 3.54 3.54 3.54 — 3.54 — bonds of (d) component (mmol/g) Mole ratio ((d)/(b)) of total numbers 0.07 0.07 0.08 — 0.07 — of carbon-carbon double bonds Mole ratio ((d + e)/(b)) of carboxy 0.13 0.18 0.20 — 0.07 0.07 groups and carboxylate groups Mole ratio ((d)/(e)) of carboxy 1.25 0.62 0.63 — — — groups and carboxylate groups Total number of moles of carboxy 0.018 0.018 0.018 — 0.018 — groups and carboxylate groups (d) Total number of moles of carboxy 0.014 0.028 0.028 — — 0.016 groups and carboxylate groups (e) Total number of moles of carboxy 0.032 0.046 0.046 — 0.018 0.016 groups and carboxylate groups ((d) + (e)) Neutralization degree (mol %) 132 125 127 149 130 140 Spherical Compression deformation amount 4.0 3.9 4.0 3.8 3.8 3.8 core (mm) Coefficient of restitution 0.005 0.010 0.012 Reference 0.010 0.000 Center hardness (Ho) (Shore C) 45 45 45 55 55 45 Surface hardness (Hs) (Shore C) 75 75 75 80 80 80 Hardness difference (Hs − Ho) 30 30 30 25 25 35 (Shore C) Golf ball Compression deformation amount 3.5 3.4 3.5 3.3 3.3 3.3 (mm) Coefficient of restitution 0.005 0.010 0.012 Reference 0.010 0.000 Flight distance on driver shot (yd) 1.7 2.4 2.7 0.0 1.4 1.0 Durability O O O O O O

Materials used in Table 1-3 are as follows.

BR730: high cis polybutadiene rubber (content of cis-1,4 bond=96 mass %; content of 1,2-vinyl bond=1.3 mass %; Moony viscosity (ML₁₊₄ (100° C.)=55; molecular weight distribution (Mw/Mn)=3) commercially available from JSR Corporation

ZN-DA90S: zinc acrylate (containing zinc stearate in an amount of 10 mass %) commercially available from Nisshoku Techno Fine Chemical Co., Ltd.

Dicumyl peroxide: commercially available from Tokyo Chemical Industry Co., Ltd.

Oleic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (unsaturated fatty acid, C18; in the chemical formula (2), R¹¹ has 8 carbon atoms, R¹² has 7 carbon atoms, (number of carbon atoms of R¹¹)/(number of carbon atoms of R¹²)=1.1)

Zinc ricinoleate: commercially available from Nitto Kasei Co., Ltd. (purity: 50 mass % (containing 50 mass % of zinc stearate)) (unsaturated fatty acid, C18; in the chemical formula (2), R¹¹ has 8 carbon atoms, R¹² has 7 carbon atoms, (number of carbon atoms of R¹¹)/(number of carbon atoms of R¹²)=1.1; R¹² has a hydroxyl group as a substituent group)

Erucic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (unsaturated fatty acid, C22; in the chemical formula (2), R^(H) has 8 carbon atoms, R¹² has 11 carbon atoms, (number of carbon atoms of R¹¹)/(number of carbon atoms of R¹²)=0.7)

Millistoleic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (unsaturated fatty acid, C14; in the chemical formula (2), R^(H) has 4 carbon atoms, R¹² has 7 carbon atoms, (number of carbon atoms of R¹¹)/(number of carbon atoms of R¹²)=0.6)

Benzoic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (purity: 98% or more)

Dimethylaminobenzoic acid: 4-dimethylaminobenzoic acid (purity: 98% or more) commercially available from Tokyo Chemical Industry Co., Ltd.

Anisic acid: p-anisic acid (4-methoxybenzoic acid, purity: 99% or more) commercially available from Tokyo Chemical Industry Co., Ltd.

Chlorobenzoic acid: 4-chlorobenzoic acid (purity: 99% or more) commercially available from Tokyo Chemical Industry Co., Ltd.

Acetoxybenzoic acid: 4-acetoxybenzoic acid (purity: 98% or more) commercially available from Tokyo Chemical Industry Co., Ltd.

2,4-dimethoxybenzoic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (purity: 99.0% or more)

2,4,5-trimethoxybenzoic acid: commercially available from. Tokyo Chemical Industry Co., Ltd. (purity: 98.0% or more)

2,4,6-trichlorobenzoic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (purity: 98.0% or more)

4-tert-butylbenzoic acid: commercially available from Tokyo Chemical Industry Co., Ltd. (purity: 99.0% or more)

Zinc oxide: “Ginrei R” commercially available from Toho Zinc Co., Ltd.

PBDS: bis(pentabromophenyl)disulfide commercially available from Kawaguchi Chemical Industry Co., Ltd.

Barium sulfate: “Barium sulfate BD” commercially available from Sakai Chemical Industry Co., Ltd.

(2) Production of Cover and Production of Golf Ball

Each of cover materials of formulations shown in Table 4 was extruded with a twin-screw kneading extruder to prepare a cover composition in a pellet form. Extruding conditions of the cover composition were a screw diameter of 45 mm, a screw rotation speed of 200 rpm, and screw L/D=35, and the mixture was heated to a temperature in a range of 160-230° C. at a position of a die of the extruder. The resulting cover composition was injection molded onto the spherical core obtained as described above to produce a golf ball having a spherical core and a cover covering the spherical core. The cover had a thickness of 1.5 mm.

TABLE 4 Cover Composition Composition Himilan 1605 50 (parts by mass) Himilan 1706 50 Titanium oxide 4 Slab hardness (Shore D) 65

Materials Used in Table 4 are as Follows.

Himilan (registered trademark) 1605: sodium ion neutralized ethylene-methacrylic acid copolymer-based ionomer resin commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1706: zinc ion neutralized ethylene-methacrylic acid copolymer-based ionomer resin commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Titanium oxide: A220 commercially available from Ishihara Sangyo Kaisha, Ltd.

Evaluation results of the golf balls are shown in Tables 1 to 3. Golf balls No. 1-No. 15 each have a spherical core that is formed from a rubber composition that contains (a) a base rubber, (b) a co-cross-linking agent, (c) a cross-linking initiator, (d) a unsaturated fatty acid and/or a metal salt thereof, and (e) an aromatic carboxylic acid and/or a metal salt thereof. These golf balls all have improved resilience and flight distance on a driver shot as compared to the golf ball No. 16.

A golf ball No. 17 does not contain the (e) component. The golf ball No. 17 has an improved flight distance on a driver shot as compared to the golf ball No. 16, but the effect is very small. A golf ball No. 18 does not contain the (d) component. The golf ball No. 18 does not have an improved resilience as compared to the golf ball No. 16, and the effect of improving a flight distance on a driver shot is small.

As a method for increasing a flight distance of a golf ball on a driver shot, for example, there are a method of using a highly resilient core and a method of using a core having a hardness distribution in which hardness increases from a center of the core toward a surface of the core. The former has an effect of increasing an initial speed of the golf ball, and the latter has an effect of increasing a launch angle and reducing spin of the golf ball. A golf ball having a higher launch angle and a lower spin has a greater flight distance.

As a technique for increasing resilience of a core, there is a method in which an organic sulfur compound is added to a rubber composition, which is a structural element of the core. Further, as a technique for increasing resilience of a core, for example, Japanese Patent Laid-Open Publication No. 2008-212681 describes a golf ball that includes a cross-linked molded product of a rubber composition as a structural element, the rubber composition containing a base rubber, a filler, an organic peroxide, an α,β-unsaturated carboxylic acid and/or a metal salt thereof as components and further containing a copper salt of a saturated or unsaturated fatty acid.

Further, as a technique for increasing a flight distance on a driver shot, Japanese Patent Laid-Open Publication No. 2013-27487 and Japanese Patent Laid-Open Publication No. 2013-27488 describe a golf ball that includes a spherical core and at least one cover covering the spherical core, the spherical core being formed from a rubber composition that contains a base rubber, an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or a metal salt thereof as a co-cross-linking agent, a cross-linking initiator, and a carboxylic acid having 4-30 carbon atoms or a carboxylic acid salt having 4-30 carbon atoms, and further contains a metal compound when only an α,β-unsaturated carboxylic acid having 3-8 carbon atoms is contained as the co-cross-linking agent.

A golf ball according to an embodiment of the present invention is excellent in flight distance on a driver shot.

A golf ball according to an embodiment of the present invention includes a spherical core and at least one cover covering the spherical core. The spherical core is formed from a rubber composition that contains (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or a metal salt thereof as a co-cross-linking agent, (c) a cross-linking initiator, (d) an unsaturated fatty acid and/or a metal salt thereof (excluding an α,β-unsaturated carboxylic acid having 3-8 carbon atoms and/or a metal salt thereof), and (e) an aromatic carboxylic acid and/or a metal salt thereof, and further contains (f) a metal compound when only an α,β-unsaturated carboxylic acid having 3-8 carbon atoms is contained as (b) the co-cross-linking agent. By using in combination (d) the unsaturated fatty acid and/or the metal salt thereof and (e) the aromatic carboxylic acid and/or the metal salt thereof in the rubber composition, a flight distance on a driver shot of the resulting golf ball is improved.

According to an embodiment of the present invention, a golf ball excellent in flight distance on a driver shot is obtained.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A golf ball, comprising: a spherical core; and a cover covering the spherical core, wherein the spherical core is formed from a rubber composition comprising a base rubber, a co-cross-linking agent, a cross-linking initiator, an unsaturated fatty acid and/or a metal salt thereof, and an aromatic carboxylic acid and/or a metal salt thereof, the co-cross-linking agent comprises at least one of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and a metal salt thereof, the unsaturated fatty acid and metal salt thereof excludes an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and a metal salt thereof, and the rubber composition further includes a metal compound when the co-cross-linking agent is consisting of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 2. The golf ball according to claim 1, wherein the rubber composition includes the unsaturated fatty acid and/or metal salt thereof and the aromatic carboxylic acid and/or metal salt thereof such that a ratio of a total number of moles of carboxy groups and carboxylate groups in the unsaturated fatty acid and/or metal salt thereof to a total number of moles of carboxy groups and carboxylate groups in the aromatic carboxylic acid and/or metal salt thereof is in a range of 0.01 to
 18. 3. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof has a number of carbon-carbon double bonds per unit mass in a range of 1.00 mmol/g to 10.00 mmol/g.
 4. The golf ball according to claim 1, wherein the rubber composition includes the unsaturated fatty acid and/or metal salt thereof and the co-cross-linking agent such that a ratio of a total number of moles carbon-carbon double bonds in the unsaturated fatty acid and/or metal salt thereof to a total number of moles of carbon-carbon double bonds of the co-cross-linking agent is in a range of 0.01 to 0.20.
 5. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is an unsaturated fatty acid having 4 to 33 carbon atoms and/or a metal salt thereof.
 6. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is an unsaturated fatty acid having one or two carbon-carbon double bonds and/or a metal salt thereof.
 7. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is an unsaturated fatty acid having 5 or more carbon atoms and having a first carbon-carbon double bond at a fourth or later carbon atom counted from a carboxy group thereof, and/or a metal salt thereof.
 8. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is an unsaturated fatty acid of formula (1) and/or a metal salt thereof,

where R¹ represents a hydrogen atom or a substituted or unsubstituted alkyl group having 1 to 25 carbon atoms, R² represents a substituted or unsubstituted alkylene group having 1 to 25 carbon atoms, R³ represents a substituted or unsubstituted alkylene group having 2 to 25 carbon atoms, m represents a natural number of 0 to 5, and when m is in a range of 2 to 5, R² is identical to or different from each other.
 9. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is a linear unsaturated fatty acid and/or a metal salt thereof.
 10. The golf ball according to claim 1, wherein the unsaturated fatty acid and/or metal salt thereof is at least one of an unsaturated fatty acid having a carbon-carbon double bond at a terminal of a hydrocarbon chain and/or a metal salt thereof, and an unsaturated fatty acid having at least one cis-isomerized carbon-carbon double bond and/or a metal salt thereof.
 11. The golf ball according to claim 1, wherein the rubber composition includes the unsaturated fatty acid and/or metal salt thereof in a range of 1 part by mass to 35 parts by mass with respect to 100 parts by mass of the base rubber.
 12. The golf ball according to claim 1, wherein the rubber composition includes the co-cross-linking agent comprising the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 13. The golf ball according to claim 1, wherein the rubber composition has a neutralization degree in a range of 100 mol % to 200 mol %.
 14. The golf ball according to claim 1, wherein the rubber composition includes an organic sulfur compound.
 15. The golf ball according to claim 14, wherein the organic sulfur compound is at least one compound selected from the group consisting of a thiophenol, a diphenyldisulfide, a thionaphthol, a thiuramdisulfide, and a metal salt thereof.
 16. The golf ball according to claim 14, wherein the rubber composition includes the organic sulfur compound in a range of 0.05 parts by mass to 5 parts by mass with respect to 100 parts by mass of the base rubber.
 17. The golf ball according to claim 1, wherein the rubber composition includes the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof in a range of 15 parts by mass to 50 parts by mass with respect to 100 parts by mass of the base rubber.
 18. The golf ball according to claim 2, wherein the unsaturated fatty acid and/or metal salt thereof has a number of carbon-carbon double bonds per unit mass in a range of 1.00 mmol/g to 10.00 mmol/g.
 19. The golf ball according to claim 2, wherein the rubber composition includes the unsaturated fatty acid and/or metal salt thereof and the co-cross-linking agent such that a ratio of a total number of moles carbon-carbon double bonds in the unsaturated fatty acid and/or metal salt thereof to a total number of moles of carbon-carbon double bonds of the co-cross-linking agent is in a range of 0.01 to 0.20.
 20. The golf ball according to claim 2, wherein the unsaturated fatty acid and/or metal salt thereof is an unsaturated fatty acid having 4 to 33 carbon atoms and/or a metal salt thereof. 